Filter media are available in a variety of materials, typically natural or synthetic polymers. The media is typically in the form of a sheet. Often this sheet is pleated to increase the amount of surface area in a given device. The side edges and ends are then sealed. The ends are typically potted into end caps that are then bonded to cartridge housings to complete the device. The ends are commonly sealed into the end caps by the use of liquid sealers such as epoxy or polyurethane, molten thermoplastics, and the like.
The sheets are thin fibrous or cast porous membranes that have about 50 to 80% of their volume in the form of voids that form the pores of the structure. Such membranes are relatively weak and fragile especially when pleated. Coarser, highly permeable layers on one or both sides of the membrane are often used to support the membrane and to maintain flow channels between the pleated plies of the membrane.
Most often, the membrane material and the potting material and/or end cap material are dissimilar. Polypropylene is a widely accepted and used material for the end cap and potting material while membranes are formed of a variety of polymers such as PVDF, nylons, PTFE, PES and other polysulphones, and the like. Because of this dissimilarity between the polymers of the different components, the membranes do not seal well to the potting materials.
The use of intermediate layers between the membrane and the potting materials is well known in order to establish a good seal. The use of epoxies and polyurethanes have been tried as well as the suggestion to use injection molding of the membrane in place in the cap. Most commonly, one uses a non-porous film laminated to the edge to form the intermediate area.
For example U.S. Pat. Nos. 4,392,958 and 4,512,892 teach the use of a separate, self-supporting, integral, non-porous film layer welded to the edges of the ends of the membrane through the use of pressure and/or heat or solvents. The film reinforces the edge and allows for a good bond between the membrane and the end sealing or potting material.
U.S. Pat. Nos. 4,929,354 and 4,906,371 teach rendering the end portions of the membrane non-porous, either by compression/heat of the edge portion of the membrane or by casting such a portion simultaneously adjacent the membrane to form an integral, non-porous edge portion. Also disclosed is a heat-sealing method in which a preformed tape with a polyethylene hot melt adhesive adheres a nylon membrane to the endcap. The process uses “heat shoes” to preheat the tape in preparation of bonding.
For use in sterilizing liquids, the completed filters are tested for integrity prior to and often after use. This is done in a variety of ways, most commonly by filling the membrane pores with a liquid such as water and measuring the flow rate of a gas, typically air through the membrane under a pressure which is a substantial portion of the membrane bubble point, for example 80% of the bubble point. For a given membrane, the bubble point and diffusion rates are known. If the measured flow is negligible, then the filter is considered to be integral. If the flow exceeds a set level, the filter fails the test. Ordinarily the higher airflow is attributed to a physical defect in the filter such as a tear, pinhole, crack, bad seal or other such large opening. However in many cases, the filter upon closer, destructive examination, such as SEM analysis, shows the device did not have a physical defect that caused the higher than allowed air flow.
The most often used membranes are hydrophilic, either those that are hydrophilic inherently or most commonly by some surface treatment means such as by coating and/or cross linking a hydrophilic coating to the surfaces of the membrane or grafting the philic functionality into the membrane surface or incorporating a hydrophilic material into the membrane material before it is made.
It is recognized in the art that any potting which involves heating of a membrane that has been rendered hydrophilic before potting may cause the hydrophilic areas under the potting material and/or adjacent to it to be weakened or lost. (See U.S. Pat. No. 4,929,354 and WO 96/14913).
This can cause several problems. First, the loss of hydrophilicity reduces the effective amount of area that a given membrane has for filtration of philic liquids. Lastly, and most importantly, this appears to interfere with the ability of the filter to pass the commonly used integrity test.
One theory is that the areas that have the loss or reduction of the philic coating revert to being hydrophobic and don't wet out completely. This allows air or the gas used in the integrity test to flow more readily on an order that typically would be considered as an indication of a physical defect. Thus, integral filters with no physical defects but some hydrophobic areas cannot be distinguished from philic filters that have physical defects such as defective seals or tears and are therefore rejected unnecessarily.
WO96/14913 suggests overcoming this issue by treating the membrane with the hydrophilic agent after either laminating a non-porous tape to the edge areas or collapsing and densifying the edge areas to render them impervious to air.
U.S. Pat. No. 6,186,341 suggests bonding a preformed thermoplastic fibrous mat along the edge of the membrane before the membrane is sealed into a thermoplastic resin which holds the membrane and cartridge pieces together. The potting material of the end cap enters the fibrous layers to form a seal with the endcap. It is stated that this method avoids the denaturing of a hydrophilic coating of the membrane in the edge area.